Floating structure

ABSTRACT

Provided is a floating structure including an upstream flow passage to pass seawater entering through a seawater inlet, a downstream flow passage to pass seawater from the upstream flow passage to guide the seawater to a seawater outlet, a connecting flow passage connecting the upstream flow passage and the downstream flow passage, and a heat exchanger provided in the connecting flow passage, the heat exchanger cooling down a heat source of the plant facility using seawater. The heat exchanger is placed above an inlet position where seawater enters the connecting flow passage from the upstream flow passage and below an outlet position where seawater exits the connecting flow passage into the downstream flow passage.

TECHNICAL FIELD

The present invention relates to a floating structure.

BACKGROUND ART

As an offshore floating structure carrying facilities such as a plant facility installed thereon, those called Floating Production Storage and Offloading (FPSO) are known in the art. Also, Floating Liquefied Natural Gas (FLNG) carrying an LNG liquefaction plant or an LNG regasification plant is known as a form of FPSO (refer to PTL 1, for example).

CITATION LIST Patent Literature (PTL 1)

Japanese Unexamined Patent Application, Publication No. 2012-122554

SUMMARY OF INVENTION Technical Problem

A plant facility carried on an FPSO such as an FLNG usually includes machines (heat sources) that generate intense heat such as a combustor or a compressor, In order to cool down the machines that generate intense heat, a cooling system that cools down the machines (the heat sources) by circulating cooling water is used. Some plant facilities carried on FPSOs use seawater as the cooling water of the cooling system.

The plant facilities are, however, installed above sea level, so that they require incidental equipment like a pump for pumping seawater, as a cooling water, up into them. The provision of such incidental equipment to a plant facility increases construction costs of the plant facility and also requires power for operating the incidental equipment.

The present invention has been made in view of the above circumstances and aims to provide a floating structure that can cool down a heat source of a plant facility installed above a sea level using seawater without requiring incidental equipment for pumping up seawater up into the plant facility.

Solution to Problem

In order to solve the foregoing problem, the following solutions have been adopted in the present invention.

The floating structure in accordance with the present invention is a floating structure that can carry a plant facility, the floating structure including a seawater inlet provided on an outer circumferential surface of the floating structure below a waterline, a seawater outlet provided on the outer circumferential surface of the floating structure below the waterline, an upstream flow passage to pass seawater entering through the seawater inlet, a downstream flow passage to pass seawater from the upstream flow passage and guide the seawater to the seawater outlet, a connecting flow passage connecting the upstream flow passage and the downstream flow passage, and a heat exchanger provided in the connecting flow passage, the heat exchanger cooling down a heat source of the plant facility using seawater, and in the floating structure, the heat exchanger is placed above an inlet position where seawater enters the connecting flow passage from the upstream flow passage and below an outlet position where seawater exits the connecting flow passage into the downstream flow passage.

The floating structure in accordance with the present invention has the seawater inlet and the seawater outlet each provided on the outer circumferential surface of the floating structure below the waterline. After entering the upstream flow passage through the seawater inlet, seawater enters the downstream flow passage via the connecting flow passage and then exits the floating structure through the seawater outlet. The connecting flow passage is provided with the heat exchanger, which cools down the heat source of the plant facility using seawater. The heat exchanger is placed above the inlet position where seawater enters the connecting flow passage from the upstream flow passage and below the outlet position where seawater exits the connecting flow passage into the downstream flow passage.

Heating seawater by the heat exchanger in the connecting flow passage induces a natural convection in which seawater is moved upward (ascends). With the natural convection, seawater flows from the inlet position below the heat exchanger toward the outlet position above the heat exchanger. The inlet position is in communication with the upstream flow passage and the outlet position is in communication with the downstream flow passage, and thus this allows the seawater to move from the upstream flow passage toward the downstream flow passage.

Thus, there is provided the floating structure that can cool down the heat source of the plant facility using seawater without requiring incidental equipment for pumping seawater up into the plant facility installed above the sea level.

In a floating structure according to a first aspect of the present invention, the seawater inlet is positioned lower than the seawater outlet.

This configuration makes smoother the movement of seawater from the upstream flow passage toward the downstream flow passage that is caused by the natural convection.

In a second aspect of the present invention, the floating structure has a cuboidal shape whose vertical length is shorter than lengths in the other directions, and the seawater inlet and the seawater outlet are provided on opposing sides, not on adjacent sides.

In this way, the seawater inlet and the seawater outlet are made sufficiently far from each other. This prevents seawater that has exited through the seawater outlet from undesirably entering the floating structure through the seawater inlet again to reduce the cooling efficiency of the heat exchanger.

In a third aspect of the present invention, the floating structure has a cuboidal shape whose vertical length is shorter than lengths in the other directions, and the seawater inlet and the seawater outlet are provided on the same side.

In this way, the upstream flow passage and the downstream flow passage can be made short as compared with the case where the seawater inlet and the seawater outlet are provided on opposing sides. Seawater is allowed to move more smoothly from the upstream flow passage toward the downstream flow passage by reducing the lengths of the upstream and downstream flow passages.

In a fourth aspect of the present invention, the floating structure has a cuboidal shape whose vertical length is shorter than lengths in the other directions, the seawater inlet is provided on an underside of the floating structure, and the seawater outlet is provided on any one side of the floating structure.

In this way, seawater enters the upstream flow passage vertically from the underside, which is positioned lowest in the outer circumferential surface of the floating structure. This makes smoother the movement of seawater from the upstream flow passage toward the downstream flow passage that is caused by the natural convection.

In a floating structure according to a fifth aspect of the present invention, the heat exchanger includes a pipe of a cooling water system that circulates cooling water cooling down the heat source of the plant facility, and exchanges heat between the cooling water circulating in the cooling water system and seawater.

This configuration enables the heat exchange between the cooling water cooling down the heat source of the plant facility and seawater, thereby appropriately cooling down the heat source of the plant facility using seawater,

A floating structure according to the present invention is a floating structure that can carry a plant facility, the floating structure including a seawater inlet provided on an outer circumferential surface of the floating structure below a waterline, a seawater outlet provided on the outer circumferential surface of the floating structure below the waterline, an upstream flow passage to pass seawater entering through the seawater inlet, a downstream flow passage to pass seawater from the upstream flow passage and guide the seawater to the seawater outlet, a connecting flow passage connecting the upstream flow passage and the downstream flow passage, and a heat exchanger provided in the connecting flow passage, the heat exchanger heating a cold source of the plant facility using seawater, and in the floating structure, the heat exchanger is placed below an inlet position where seawater enters the connecting flow passage from the upstream flow-passage and above an outlet position where seawater exits the connecting flow passage into the downstream flow passage,

The floating structure in accordance with the present invention has the seawater inlet and the seawater outlet each provided on the outer circumferential surface of the floating structure below the waterline. After entering the upstream flow passage through the seawater inlet, seawater enters the downstream flow passage via the connecting flow passage and then exits the floating structure through the seawater outlet. The connecting flow passage is provided with the heat exchanger, which heats the cold source of the plant facility using seawater. The heat exchanger is placed below the inlet position where seawater enters the connecting flow passage from the upstream flow passage and above the outlet position where seawater exits the connecting flow passage into the downstream flow passage.

Cooling down seawater by the heat exchanger in the connecting flow passage induces a natural convection in which seawater is moved downward (descends). With the natural convection, seawater flows from the inlet position above the heat exchanger toward the outlet position below the heat-exchanger. The inlet position is in communication with the upstream flow passage and the outlet position is in communication with the downstream flow passage, and thus this allows the seawater to move from the upstream flow passage toward the downstream flow passage.

Thus, there is provided the floating structure that can heat the cold source of the plant facility using seawater without requiring incidental equipment for pumping seawater up into the plant facility installed above the sea level.

The above floating structure according to the present invention may be configured such that the floating structure can carry a regasification plant facility for liquefied gas and the heat exchanger gasifies the liquefied gas by heating the liquefied gas using seawater.

In accordance with the configuration, there is provided a floating structure that can gasify liquefied gas by heating the liquefied gas using seawater.

Advantageous Effects of Invention

According to the present invention, there can be provided a floating structure that can cool down a heat source of a plant facility installed above the sea level using seawater without requiring incidental equipment for pumping up the seawater up into the plant facility.

According to the present invention, there can also be provided a floating structure that can heat a cold source of the plant facility installed above the sea level using seawater without requiring incidental equipment for pumping up the seawater up into the plant facility.

BRIEF DESCRIPTION OF DRAWINGS (FIG. 1)

FIG. 1 is a perspective view of a floating structure of a first embodiment.

(FIG. 2)

FIG. 2 shows a schematic configuration of the floating structure and a cooling water system of the first embodiment.

(FIG. 3)

FIG. 3 shows a schematic configuration of a floating structure and a cooling water system of a second embodiment.

(FIG. 4)

FIG. 4 shows a schematic configuration of a floating structure and a cooling water system of a third embodiment.

(FIG. 5)

FIG. 5 shows a schematic configuration of a floating structure and an intermediate heating medium system of a fourth embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

Hereinafter, a floating structure of a first embodiment of the present invention will be described while referring to FIGS. 1 and 2.

FIG. 1 is a perspective view showing a floating structure 100 of the embodiment. FIG. 2 shows a schematic configuration of the floating structure 100 and a cooling water system 101 of the embodiment.

The floating structure 100 shown in FIG. 1 is a structure called Floating Production Storage and Offloading (FPSO), and can carry a plant facility P. The plant facility P includes various power sources such as a compressor, and the power sources are heat sources generating heat. The plant facility P is provided with the cooling water system 101 (refer to FIG. 2) in which cooling water circulates in order to cool down the plurality of power sources as the heat sources. The plant facility P may be any of various facilities equipped with heat sources, including an LNG liquefaction plant facility, an LNG regasification plant facility, CO₂ capture equipment, and a power generating facility.

As shown in FIG. 1, the floating structure 100 is a cuboidal structure floating on the sea. The floating structure 100 has horizontal lengths of L1 and L2 and a vertical length of L3. The vertical length L3 is shorter the horizontal lengths L1 and L2.

As shown in FIG. 1, a waterline D is the line where the four sides of the floating structure 100 meet the sea level S. The floating structure 100 has a seawater inlet 10 below the waterline D in the sea. A seawater outlet 20 (refer to FIG. 2) is provided on a side of the floating structure 100 opposed to the side provided with the seawater inlet 10. In this way, the seawater inlet 10 and the seawater outlet 20 are provided on the opposing sides, not on adjacent sides.

FIG. 2 shows the schematic configuration of the floating structure 100 and the cooling water system 101 in the embodiment. In FIG. 2, the floating structure 100 is shown in a cross section taken along a direction of the length L1 in FIG. 1. The cross section is taken through the seawater inlet 10 and the seawater outlet 20.

As shown in FIG. 2, the seawater inlet 10 and the seawater outlet 20 are provided on an outer circumferential surface of the floating structure 100 below the waterline D. The floating structure 100 has an upstream flow passage 70 to pass seawater entering through the seawater inlet 10 and a downstream flow passage 80 to pass seawater from the upstream flow passage 70 and guide the seawater to the seawater outlet 20. The upstream flow passage 70 and the downstream flow passage 80 are connected by a connecting flow passage 90.

The seawater inlet 10 is provided on a side of the floating structure 100 and has a rectangular shape as seen orthogonally to the side, as shown in FIG. 1. The upstream flow passage 70, the downstream flow passage 80, and the connecting flow passage 90 have a rectangular cross section (not shown) similar to the shape of the seawater inlet 10. The cross-sectional shape of the upstream flow passage 70, the downstream flow passage 80, and the connecting flow passage 90 may be, for example, a circle or an ellipse, not a rectangular.

The connecting flow passage 90 is provided with a heat exchanger 30 that cools down a heat source (not shown) of the plant facility P using seawater. The heat exchanger 30 includes a pipe to pass the cooling water circulating in the cooling water system 101. A shell and tube heat exchanger or a plate heat exchanger may be used as the heat exchanger 30. The heat exchanger 30 does not directly cool down the heat-source of the plant facility P, but indirectly cools down the heat source of the plant facility P by cooling down the cooling water circulating in the cooling water system 101 that cools down the heat source, as will be described later.

The cooling water system 101 is provided with an upstream header 51, a downstream header 52, a cooler 61, a cooler 62, and a cooler 63. The cooling water in the upstream header 51 diverges into the cooler 61, the cooler 62, and the cooler 63. The coolers 61 to 63 are provided such that they are directly or indirectly in contact with a plurality of heat sources (not shown) of the plant facility P.

After passing through the coolers 61 to 63, the cooling water is raised in temperature due to the heat exchange with the heat sources of the plant facility P. The cooling wafer is sent by a circulation pump 40 through a cooling water pipe 102 to the pipe forming a part of the heat exchanger 30. After cooled down by seawater in the heat exchanger 30, the cooling water is again supplied through a cooling water pipe 103 to the upstream header 51. Thus, the cooling water in the cooling water system 101 is circulated by the power of the circulation pump 40.

When the heat exchanger 30 exchanges heat between the cooling water of the cooling water system 101 and seawater to heat the seawater, this induces a natural convection in which seawater is moved upward. With the natural convection, the seawater flows from an inlet position below the heat exchanger 30 toward an outlet position above the heat exchanger 30, The inlet position is in communication with the upstream flow passage 70 and the outlet position is in communication with the downstream flow passage 80, so that the seawater moves from the upstream flow passage toward the downstream flow passage.

Seawater starts to move from the seawater inlet 10 toward the connecting flow passage 90 in the upstream flow passage 70, as the seawater moves upward in the connecting flow passage 90, Similarly, seawater starts to move from the connecting flow passage 90 toward the seawater outlet 20 in the downstream flow passage 80, as the seawater moves upward in the connecting flow passage 90. In this way, the heat-exchange by the heat exchanger 30 between the cooling water of the cooling water system 101 and seawater induces the natural convection of seawater from the seawater inlet 10 toward the seawater outlet 20,

As shown in FIG. 2, the seawater inlet 10 of the embodiment is positioned lower than the seawater outlet 20. The upstream flow passage 70 and the downstream flow passage 80 each extends horizontally so as to have a constant vertical distance from the waterline D.

Because of such a structure, the upward movement (the natural convection) of seawater in the connecting flow passage 90 allows smooth movement of seawater from the seawater inlet 10 to the connecting flow passage 90 and that from the connecting flow passage 90 to the seawater outlet 20.

As described above, the floating structure 100 of the embodiment has the seawater inlet 10 and the seawater outlet 20 each provided on the outer circumferential surface of the floating structure 100 below the waterline D. After entering the upstream flow passage 70 through the seawater inlet 10, the seawater enters the downstream flow passage 80 via the connecting flow passage 90 and then exits the floating structure 100 through the seawater outlet 20. The connecting flow passage 90 is provided with the heat exchanger 30, which cools down the heat sources (not shown) of the plant facility P using seawater. The heat exchanger 30 is placed above the inlet position where seawater enters the connecting flow passage 90 from the upstream flow passage 70 and below the outlet position where seawater exits the connecting flow passage 90 into the downstream flow passage 80.

Heating seawater by the heat exchanger 30 in the connecting flow passage 90 induces the natural convection in which seawater is moved upward. With the natural convection, seawater flows from the inlet position below the heat exchanger 30 toward the outlet position above the heat exchanger 30. The inlet position is in communication with the upstream flow passage 70 and the outlet position is in communication with the downstream flow passage 80, and thus this allows the seawater to move from the upstream flow passage 70 toward the downstream flow passage 80.

Thus, there is provided the floating structure 100 that can cool down the heat sources of the plant facility P using seawater without requiring incidental equipment for pumping seawater up into the plant facility installed above the sea level S.

In the floating structure 100 of the embodiment, the seawater inlet 10 is positioned lower than the seawater outlet 20. This configuration makes smoother the movement of the seawater from the upstream flow passage 70 toward the downstream flow passage 80 that is caused by the natural convection,

The floating structure 100 of the embodiment has the cuboidal shape whose vertical length L3 is shorter than the lengths L1 and L2 in the other directions, and has the seawater inlet 10 and the seawater outlet 20 on opposing sides, not on adjacent sides. In this way, the seawater inlet 10 and the seawater outlet 20 are made sufficiently far from each other. This prevents seawater that has exited through the seawater outlet 20 from undesirably entering the floating structure 100 through the seawater inlet 10 again to reduce the cooling efficiency of the heat exchanger 30.

Second Embodiment

Hereinafter, a floating structure of a second embodiment of the present invention will be described while referring to FIG. 3.

The second embodiment is a modification of the first embodiment, and is similar to the first embodiment unless otherwise described hereinafter. Components denoted by the same reference numerals as those in the first embodiment will not be explained in the second embodiment.

The floating structure 100 of the first embodiment and a floating structure 200 of the second embodiment are different in position and shape of the flow passages from the seawater inlet to the seawater outlet.

As shown in FIG. 3, the floating structure 200 of the second embodiment has a seawater inlet 11 and a seawater outlet 21 on the same side. The floating structure 200 has an upstream flow passage 71 to pass seawater entering through the seawater inlet 11 and a downstream flow passage 81 to pass seawater from the upstream flow passage 71 to guide the seawater to the seawater outlet 21. The upstream flow passage 71 and the downstream flow passage 81 are connected by a connecting flow passage 91.

Because the floating structure 200 of the second embodiment has the seawater inlet 11 and the seawater outlet 21 on the same side, the upstream flow passage and the downstream flow passage can be made shorter than those in the case where the seawater inlet and the seawater outlet are provided on opposing sides. Seawater is allowed to move more smoothly from the upstream flow passage toward the downstream flow passage by reducing the lengths of the upstream and downstream flow passages.

(Third Embodiment)

Hereinafter, a floating structure of a third embodiment of the present invention will be described while referring to FIG. 4.

The third embodiment is a modification of the first embodiment, and is similar to the first embodiment unless otherwise described hereinafter. Components denoted by the same reference numerals as those in the first embodiment will not be explained in the third embodiment.

The floating structure 100 of the first embodiment and a floating structure 300 of the third embodiment are different in position and shape of the flow passages from the seawater inlet to the seawater outlet,

As shown in FIG. 4, the floating structure 300 of the third embodiment has a seawater inlet 12 on an underside of the floating structure 300 and a seawater outlet 22 on any one side of the floating structure 300. The floating structure 300 has an upstream flow passage 72 to pass seawater entering through the seawater inlet 12 and a downstream flow passage 82 to pass seawater from the upstream flow passage 72 to guide the seawater to the seawater outlet 22, The upstream flow passage 72 and the downstream flow passage 82 are connected by a connecting flow passage 92.

Because the floating structure 300 of the third embodiment has the seawater inlet 12 on the underside of the floating structure 300, seawater enters the upstream flow passage 72 vertically from the underside, which is positioned lowest in the outer circumferential surface of the floating structure 300. This makes smoother the movement of the seawater from the upstream flow passage 72 toward the downstream flow passage 82 that is caused by the natural convection.

In addition, the upstream flow passage and the downstream flow passage can be made shorter than those in the case where the seawater inlet and the seawater outlet are provided on opposing sides. Furthermore, because seawater enters from the underside of the floating structure 300, seawater that is lower in temperature than that in the other embodiments can be guided to the heat exchanger 30.

Fourth Embodiment

Hereinafter, a floating structure in a fourth embodiment of the present invention will be described using FIG. 5. FIG. 5 shows a schematic configuration of a floating structure and an intermediate heating medium system of the fourth embodiment.

The heat exchanger 30 of the floating structure 100 of the first embodiment exchanges heat between seawater and the cooling water circulating in the cooling water system 101 of the plant facility P. Meanwhile, a heat exchanger 430 of a floating structure 400 in the embodiment exchanges heat between liquefied gas and seawater.

The embodiment is a modification of the first embodiment, and is similar to the first embodiment unless otherwise described hereinafter. Components denoted by the same reference numerals as those in the first embodiment will not be explained in the embodiment.

A plant facility P carried on the floating structure 400 of the fourth embodiment is a regasification plant facility that regasifies liquefied natural gas (LNG). A temperature difference between LNG and seawater is about 200° C., and accordingly, LNG can be regasified by using seawater as a heat source for heating.

In the first embodiment, the cooling water system 101 is used for circulating the cooling water that exchanges heat with seawater. The fourth embodiment uses an intermediate heating medium system 401 for circulating an intermediate heating medium such as propane gas, instead of the cooling water system 101. The intermediate heating medium circulates in the intermediate heating medium system 401 as a heating medium that transfers heat of seawater to the LNG.

In the first embodiment, the coolers 61, 62, and 63 are used to directly or indirectly cool down the plurality of heat sources of the plant facility P. In the fourth embodiment, the plant facility P have heaters 461, 462, and 463 instead of the coolers 61, 62, and 63, and the heaters 461, 462, and 463 heat the liquefied gas by using the intermediate heating medium heated by seawater. The liquefied gas is gasified when heated by the heaters 461, 462, and 463.

The heat exchanger 30 in the first embodiment exchanges heat between seawater and the cooling water that is higher in temperature than the seawater, thereby heating the seawater. Meanwhile, a heat exchanger 430 in the fourth embodiment exchanges heat between seawater and the intermediate heating medium that is lower in temperature than the seawater, thereby cooling down the seawater. Accordingly, in the embodiment, the natural convection of seawater moving downward is induced in the vicinity of the heat exchanger 430.

The seawater inlet 10 in the first embodiment is positioned lower than the seawater outlet 20. In contrast, a seawater inlet 13 of the embodiment is positioned higher than a seawater outlet 23, An upstream flow passage 73 and a downstream flow passage 83 each extends horizontally so as to have a constant vertical distance from the waterline D.

Thus, the heat exchanger 430 of the embodiment is placed below an inlet position where seawater enters a connecting flow passage 93 from the upstream flow passage 73 and above an outlet position where seawater exits the connecting flow passage 93 into the downstream flow passage 83.

Because of such a structure, the downward movement (the natural convection) of seawater in the connecting flow passage 93 allows smooth movement of seawater from the seawater inlet 13 to the connecting flow passage 93 and that from the connecting flow passage 93 to the seawater outlet 23.

As described above, the fourth embodiment is the modification of the first embodiment, although it may be configured otherwise. For example, the fourth embodiment may be a modification of the second embodiment. In that case, the heat exchanger 430 exchanges heat between seawater and the intermediate heating medium that is lower in temperature than the seawater, thereby cooling down the seawater, similarly to the above description. In the modification, the seawater inlet 13 and the seawater outlet 23 are provided on the same side, and the seawater outlet 23 is positioned lower than the seawater inlet 13.

Furthermore, the fourth embodiment may be a modification of the third embodiment. In that case, the heat exchanger 430 exchanges heat between seawater and the intermediate heating medium that is lower in temperature than the seawater, thereby cooling down the seawater, similarly to the above description. In the modification, the seawater outlet 23 is provided on an underside of the floating structure 400, and the seawater inlet 13 is provided on any one side of the floating structure 400.

While preferable embodiments of the present invention have been described above, other embodiments are possible that are changed appropriately without departing from the technical idea of the present invention.

REFERENCE SIGNS LIST

-   10,11,12,13 Seawater inlet -   20,21,22,23 Seawater outlet -   30, 430 Heat exchanger -   40 circulation pump -   51 Upstream header -   52 Downstream header -   61, 62, 63 Cooler -   70,71,72,73 Upstream flow passage -   80, 81, 82, 83 Downstream flow passage -   90, 91, 92, 93 Connecting flow passage -   100, 200, 300, 400 Floating structure -   101 Cooling water system -   102, 103 Cooling water pipe -   401 Intermediate heating medium system -   461, 462, 463 Heater -   D Waterline -   L1 Floating structure length -   L2 Floating structure width -   L3 Floating structure height (vertical length of floating structure) -   P Plant facility -   S Sea level 

1. A floating structure that can carry a plant facility, the floating structure comprising: a seawater inlet provided on an outer circumferential surface of the floating structure below a water line; a seawater outlet provided on the outer circumferential surface of the floating structure below the waterline; an upstream flow passage to pass seawater entering through the seawater inlet; a downstream flow passage to pass seawater from the upstream flow passage and guide the seawater to the seawater outlet; a connecting flow passage connecting the upstream flow passage and the downstream flow passage; and a heat exchanger provided in the connecting flow passage, the heat exchanger cooling down a heat source of the plant facility using seawater. wherein the heat exchanger is placed above an inlet position where seawater enters the connecting flow passage from the upstream flow passage and below an outlet position where seawater exits the connecting flow passage into the downstream flow passage.
 2. The floating structure according to claim 1, wherein the seawater inlet is positioned lower than the seawater outlet.
 3. The floating structure according to claim 1, wherein the floating structure has a cuboidal shape whose vertical length is shorter than lengths in other directions, and the seawater inlet and the seawater outlet are provided on opposing sides, not on adjacent sides.
 4. The floating structure according to claim 1, wherein the floating structure has a cuboidal shape whose vertical length is shorter than lengths in other directions, and the seawater inlet and the seawater outlet are provided on the same side.
 5. The floating structure according to claim 1, wherein the floating structure has a cuboidal shape whose vertical length is shorter than lengths in other directions, the seawater inlet is provided on an underside of the floating structure, and the seawater outlet is provided on any one side of the floating structure.
 6. The floating structure according to claim 1, wherein the heat exchanger includes a pipe of a cooling water system that circulates cooling water cooling down the heat source of the plant facility, and exchanges heat between the cooling water circulating in the cooling water system and seawater.
 7. A floating structure that can carry a plant facility, the floating structure comprising: a seawater inlet provided on an outer circumferential surface of the floating structure below a waterline; a seawater outlet provided on the outer circumferential surface of the floating structure below the waterline; an upstream flow passage to pass seawater entering through the seawater inlet; a downstream flow passage to pass seawater from the upstream flow passage and guide the sea water to the sea water outlet; a connecting flow passage connecting the upstream flow passage and the downstream flow passage; and a heat exchanger provided in the connecting flow passage, the heat exchanger heating a cold source of the plant facility using seawater, wherein the heat exchanger is placed below an inlet position where seawater enters the connecting flow passage from the upstream flow passage and above an outlet position where seawater exits the connecting flow passage into the downstream flow passage.
 8. The floating structure according to claim 7, wherein the floating structure can carry a regasification plant facility for liquefied gas, and the heat exchanger gasifies the liquefied gas by heating the liquefied gas using sea water. 